1. Field of the Invention
This invention relates to the attenuation and reduction of machinery and gas flow noise and turbulence in duct systems including, but not restricted to, heating, ventilating and air conditioning air duct systems.
2. Background Information
Conventional sound attenuators used in duct systems use resistance provided by filler material when sound travels through pores of the filler material. Typically FIBERGLASS, ROCKWOOL, foam and other fibrous materials are used for this purpose. Perforated sheets are used to increase the access of the sound from flow passage to filler material. The filler material that is used to attenuate sound creates some new problems. At higher gas flow velocities the filler material gets eroded into small particles and gets entrained in airflow contaminating indoor air of a facility. The filler materials produce some toxic gases, cause microorganisms to grow or release some hazardous products when they come in contact with some other chemicals. These problems make the use of filler material in sound attenuators dangerous. Pat. No. 4,287,962 Packless Silencer, Ingard et al, Sep. 8, 1981 addresses the above mentioned problems associated with the filler material of fibrous nature. Ingard et al uses sound attenuators with acoustic resistance provided by resistive sheets or perforated face sheets. While sound attenuators having perforated sheets were an improvement, they did not include benefits that round and oval passages inherently have over flat sheets or rectangular shapes. The entrance and exit were not designed optimally and this causes flow noise to increase and often results in turbulence of flow. Turbulent flow increases the energy required to maintain gas flow. These disadvantages led to a less than optimal acoustical and flow performance. As will be seen in the subsequent description, the present invention overcomes these disadvantages of the prior art.